We have devised a new method to measure cytotoxic T cell activity in vitro and perfected a method to measure immune rejection in vivo. Using these techniques, in combination with our standard thymic organ culture, we are in the process of studying four questions. 1.Do newly born T cells leave the thymus in a tolerizable or activateable state. 2.Does thymic epithelium present antigens in a tolerogenic or immunogenic fashion. 3.Does T cell priming and/or memory require the presence of B cells. 4.What is the frequency of alloreactive T cells. Our preliminary experiments suggest that newly developed CD4+ and CD8+ T cells are fully capable of responding to allogeneic stimulators, thus refuting models that require the assumption that T cells leave the thymus in an anergizable state.